We tested the hand muscles of 115 normal subjects electromyographically to determine the function of these muscles in power grip and precision handling. In power grip all the intrinsics and extrinsics were tested in detail (ten subjects per muscle); in precision handling the intrinsics of the thumb and first two fingers were tested; other intrinsics and extrinsics were spot-checked. In the experimental laboratory, activities were developed to represent resisted motions performed by the hand in activities of daily living. Graded resistances were tested and various sizes of simulated objects were used. The classifications of motion were: (1) power grip, including squeeze, disc, hook, and spherical grips, (2) precision handling, including rotation and translation, and (3) pinch. Our findings warrant the following conclusions: 1. In power grip the extrinsics provide the major gripping force. All of the extrinsics are involved in power gripping and are used in proportion to the desired force to be used against the external force. The major intrinsic muscles of power grip are the interossei, used as phalangeal rotators and metacarpophalangeal flexors. The lumbricales, with the exception of the fourth, are not significantly used in power grip. The thenar muscles are used in all forms of power grip except hook grip. 2. In precision handling, specific extrinsic muscles provide gross motion and compressive forces. In rotation motions the interossei are important in imposing the necessary rotational forces on the object to be rotated; the motion of the metacarpophalangeal joint which provides this rotation is abduction or adduction, not rotation of the first phalanx. The lumbricales are interphalangeal joint extensors as in the unloaded hand, and additionally are first phalangeal abductor-adductors and rotators. In translation motions towards the palm, the interossei provide intrinsic compression and rotation forces for most efficient finger positioning; the lumbricalis is not active. Moving away from the palm the handled object is driven by interossei and lumbricales to provide intrinsic compression and metacarpophalangeal-joint flexion and interphalangeal-joint extension. The thenar muscles in precision handling act as a triad of flexor pollicis brevis, opponens pollicis, and abductor pollicis brevis to provide adduction across the palm, internal rotation of the first metacarpal, and maintenance of web space depth. The adductor pollicis is used in specific situations when force is required to adduct the first metacarpal towards the second. 3. In pinch, compression is provided primarily by the extrinsic muscles. Phalangeal rotational position is adjusted by the interossei and perhaps also by the lumbricales. Compression is assisted by the metacarpophalangeal-joint flexion force of the interossei and flexor pollicis brevis and by the adducting force of the adductor pollicis. The opponens assists through rotational positioning of the first metacarpal.